Network Working Group E. Ivov
Internet-Draft Atlassian
Intended status: Standards Track E. Rescorla
Expires: January 17, 2018 RTFM, Inc.
J. Uberti
Google
P. Saint-Andre
Filament
July 16, 2017
Trickle ICE: Incremental Provisioning of Candidates for the Interactive
Connectivity Establishment (ICE) Protocol
draft-ietf-ice-trickle-13
Abstract
This document describes "Trickle ICE", an extension to the
Interactive Connectivity Establishment (ICE) protocol that enables
ICE agents to send and receive candidates incrementally rather than
exchanging complete lists. With such incremental provisioning, ICE
agents can begin connectivity checks while they are still gathering
candidates and considerably shorten the time necessary for ICE
processing to complete.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 17, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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candidates as soon as an ICE session has been initiated and a
candidate has become available. Connectivity checks for a media
stream can also start as soon as the first candidates for that stream
become available.
Trickle ICE can reduce session establishment times in cases where
connectivity is confirmed for the first exchanged candidates (e.g.,
where candidates for one of the agents are directly reachable from
the second agent, such as candidates at a media relay). Even when
this is not the case, performing candidate gathering for both agents
and connectivity checks in parallel can considerably shorten ICE
processing times.
It is worth noting that there is quite a bit of operational
experience with the Trickle ICE technique, going back as far as 2005
(when the XMPP Jingle extension defined a "dribble mode" as specified
in [XEP-0176]); this document incorporates feedback from those who
have implemented and deployed the technique.
In addition to the basics of Trickle ICE, this document also
describes how to discover support for Trickle ICE, how regular ICE
processing needs to be modified when building and updating check
lists, and how Trickle ICE implementations interoperate with agents
that only implement regular ICE processing as defined in
[rfc5245bis].
This specification does not define the usage of Trickle ICE with any
specific signaling protocol (however, see
[I-D.ietf-mmusic-trickle-ice-sip] for usage with SIP [RFC3261] and
[XEP-0176] for usage with XMPP [RFC6120]). Similarly, it does not
define Trickle ICE in terms of the Session Description Protocol (SDP)
[RFC4566] or the offer/answer model [RFC3264] because the technique
can be and already is used in application protocols that are not tied
to SDP or to offer/answer semantics. However, because SDP and the
offer/answer model are familiar to most readers of this
specification, some examples in this document use those particulars
in order to explain the underlying concepts.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification makes use of all terminology defined for
Interactive Connectivity Establishment in [rfc5245bis]. In addition,
it defines the following terms:
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Generation: All of the candidates conveyed within an ICE session;
these are the candidates that are associated with a specific
local/remote ufrag pair (which will change on ICE restart, if any
occurs).
ICE Description: Any session-related (as opposed to candidate-
related) attributes required to configure an ICE agent. These
include but are not limited to the username fragment, password,
and other attributes.
Trickled Candidates: Candidates that a Trickle ICE agent conveys
after conveying the initial ICE description or responding to the
initial ICE description, but within the same ICE session.
Trickled candidates can be conveyed in parallel with candidate
gathering and connectivity checks.
Trickling: The act of conveying trickled candidates.
Half Trickle: A Trickle ICE mode of operation where the initiator
gathers a full generation of candidates strictly before creating
and conveying the initial ICE description. Once conveyed, this
candidate information can be processed by regular ICE agents,
which do not require support for this specification. It also
allows Trickle ICE capable responders to still gather candidates
and perform connectivity checks in a non-blocking way, thus
roughly providing "half" the advantages of Trickle ICE. The
mechanism is mostly meant for use in cases where the responder's
support for Trickle ICE cannot be confirmed prior to conveying the
initial ICE description.
Full Trickle: The typical mode of operation for Trickle ICE agents,
in which the initial ICE description can include any number of
candidates (even zero candidates) and does not need to include a
full generation of candidates as in half trickle.
3. Determining Support for Trickle ICE
To fully support Trickle ICE, applications SHOULD incorporate one of
the following mechanisms to enable implementations to determine
whether Trickle ICE is supported:
1. Provide a capabilities discovery method so that agents can verify
support of Trickle ICE prior to initiating a session (XMPP's
Service Discovery [XEP-0030] is one such mechanism).
2. Make support for Trickle ICE mandatory so that user agents can
assume support.
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If an application protocol does not provide a method of determining
ahead of time whether Trickle ICE is supported, agents can make use
of the half trickle procedure described in Section 14.
Prior to conveying the initial ICE description, agents using
signaling protocols that support capabilities discovery can attempt
to verify whether or not the remote party supports Trickle ICE. If
an agent determines that the remote party does not support Trickle
ICE, it MUST fall back to using regular ICE or abandon the entire
session.
Even if a signaling protocol does not include a capabilities
discovery method, a user agent can provide an indication within the
ICE description that it supports Trickle ICE using a token of
"trickle" in the ice-options attribute. This token MUST be provided
either at the session level or, if at the media stream level, for
every media stream (an agent MUST NOT specify Trickle ICE support for
some media streams but not others).
Dedicated discovery semantics and half trickle are needed only prior
to session initiation. After a session is established and Trickle
ICE support is confirmed for both parties, either agent can use full
trickle for subsequent exchanges.
4. Conveying the Initial ICE Description
An initiator can start gathering candidates as soon as it has an
indication that communication is imminent (e.g., a user interface cue
or an explicit request to initiate a session). Unlike in regular
ICE, in Trickle ICE implementations do not need to gather candidates
in a blocking manner. Therefore, unless half trickle is being used,
the initiator SHOULD generate and transmit their initial ICE
description as early as possible, so that the remote party can start
gathering and trickling candidates.
An initiator MAY include any mix of candidates when conveying the
initial ICE description. This includes the possibility of conveying
all the candidates the initiator plans to use (as in half trickle
mode), conveying only a publicly-reachable IP address (e.g., a
candidate at a media relay that is known to not be behind a
firewall), or conveying no candidates at all (in which case the
initiator can obtain the responder's initial candidate list sooner
and the responder can begin candidate gathering more quickly).
Methods for calculating priorities and foundations, as well as
determining redundancy of candidates, work just as with regular ICE
(with the exception of pruning of duplicate peer reflexive candidates
as described under Section 5.2).
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When a responder receives the initial ICE description, it will first
check if the ICE description or initiator indicates support for
Trickle ICE as explained in Section 3. If this is not the case, the
responder MUST process the initial ICE description according to
regular ICE procedures [rfc5245bis] (or, if no ICE support is
detected at all, according to relevant processing rules for the
underlying signaling protocol, such as offer/answer processing rules
[RFC3264]).
If support for Trickle ICE is confirmed, a responder will
automatically assume support for regular ICE as well. Specifically,
the rules from [rfc5245bis] would imply that ICE itself is not
supported if the initial ICE description includes no candidates;
however, such a conclusion is not warranted if the responder can
confirm that the initiator supports Trickle ICE; in this case,
fallback to [RFC3264] is not necessary.
If the initial ICE description indicates support for Trickle ICE, the
responder will determine its role and start gathering and
prioritizing candidates; while doing so, it will also respond by
conveying its own ICE description, so that both the initiator and the
responder can start forming check lists and begin connectivity
checks.
5.1. Conveying the Initial Response
A responder can respond to the initial ICE description at any point
while gathering candidates. Here again the ICE description MAY
contain any set of candidates, including all candidates or no
candidates. (The benefit of including no candidates is to convey the
ICE description as quickly as possible, so that both parties can
consider the overall session to be under active negotiation as soon
as possible.)
As noted in Section 3, in application protocols that use SDP the
responder's ICE description can indicate support for Trickle ICE by
including a token of "trickle" in the ice-options attribute.
5.2. Forming Check Lists and Beginning Connectivity Checks
As soon as the agents have obtained local and remote candidates, both
agents begin forming candidate pairs, computing candidate pair
priorities, ordering candidate pairs, pruning duplicate pairs, and
creating check lists according to regular ICE procedures
[rfc5245bis].
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According to those procedures, in order for candidate pairing to be
possible and for duplicate candidates to be pruned, the candidates
would need to be provided in the relevant ICE descriptions. Under
Trickle ICE, check lists can be empty until candidates are conveyed
or received. Therefore Trickle ICE agents handle check lists and
candidate pairing in a slightly different way than regular ICE
agents: the agents still create the check lists, but they populate
the check lists only after they actually have the candidate pairs.
A Trickle ICE agent initially considers all check lists to be frozen.
It then inspects the first check list and attempts to unfreeze all
candidate pairs it has received so far that belong to the first
component on the first media stream (i.e., the first media stream
that was reported to the ICE implementation from the using
application). If that first component of the first media stream does
not contain candidates for one or more of the currently known pair
foundations, and if candidate pairs already exist for that foundation
in one of the following components or media streams, then the agent
unfreezes the first of those candidate pairs.
With regard to pruning of duplicate candidate pairs, a Trickle ICE
agent SHOULD follow a policy of keeping the higher priority candidate
unless it is peer reflexive.
6. Initiator Procedures
When processing the initial ICE description from a responder, the
initiator follows regular ICE procedures to determine its role, after
which it forms check lists (as described in Section 5.2) and begins
connectivity checks.
7. Performing Connectivity Checks
For the most part, Trickle ICE agents perform connectivity checks
following regular ICE procedures. However, the fact that gathering
and communicating candidates is asynchronous in Trickle ICE imposes a
number of changes as described in the following sections.
7.1. Scheduling Checks
The ICE specification [rfc5245bis], Section 5.1.4, requires that an
agent will terminate the timer for a triggered check in relation to
an active check list once the agent has exhausted all frozen pairs in
the check list. This will not work with Trickle ICE, because more
pairs will be added to the check list incrementally.
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Therefore, a Trickle ICE agent SHOULD NOT terminate the timer until
the state of the check list is Completed or Failed as specified
herein (see Section 8.2).
7.2. Check List and Timer State Updates
The ICE specification [rfc5245bis], Section 6.2.5.3.3, requires that
agents update check lists and timer states upon completing a
connectivity check transaction. During such an update, regular ICE
agents would set the state of a check list to Failed if both of the
following two conditions are satisfied:
o all of the pairs in the check list are either in the Failed state
or Succeeded state; and
o there is not a pair in the valid list for each component of the
media stream.
With Trickle ICE, the above situation would often occur when
candidate gathering and trickling are still in progress, even though
it is quite possible that future checks will succeed. For this
reason, Trickle ICE agents add the following conditions to the above
list:
o all candidate gathering has completed and the agent is not
expecting to discover any new local candidates;
o the remote agent has conveyed an end-of-candidates indication for
that check list as described in Section 8.2.
When a check list is set to Failed as described above, regular ICE
requires the agent to update all other check lists, placing one pair
from each check list into the Waiting state - effectively unfreezing
all remaining check lists. However, under Trickle ICE other check
lists might still be empty at this point (because candidates have not
yet been received), and following only the rules from regular ICE
would prevent the agent from unfreezing those check lists (because
the state of a check list depends on the state of the candidate pairs
in that check list, but there are none yet). Therefore a Trickle ICE
agent needs to monitor whether a check list is active or frozen
independently of the state of the candidate pairs in the check list
(since there might not be any pairs yet). With regard to empty check
lists, by default a Trickle ICE agent MAY consider an empty check
list to be either active or frozen. When a Trickle ICE agent
considers an empty check list to be frozen, during the candidate
checking process it SHOULD change the check list to active if
checking of another check list is completely finished (i.e., if every
pair in the other check list is either Successful or Failed), if
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another check list has a valid candidate pair for all components, or
if it adds a candidate pair to the check list (because, in accordance
with Section 8.1.1, when inserting a new candidate pair into an empty
check list, the agent sets the pair to a state of Waiting).
8. Discovering and Conveying Additional Local Candidates
After candidate information has been conveyed, agents will most
likely continue discovering new local candidates as STUN, TURN, and
other non-host candidate gathering mechanisms begin to yield results.
Whenever an agent discovers such a new candidate it will compute its
priority, type, foundation and component ID according to regular ICE
procedures.
The new candidate is then checked for redundancy against the existing
list of local candidates. If its transport address and base match
those of an existing candidate, it will be considered redundant and
will be ignored. This would often happen for server reflexive
candidates that match the host addresses they were obtained from
(e.g., when the latter are public IPv4 addresses). Contrary to
regular ICE, Trickle ICE agents will consider the new candidate
redundant regardless of its priority.
Next the agent "trickles" the newly discovered candidate(s) to the
remote agent. The actual delivery of the new candidates is handled
by a signaling protocol such as SIP or XMPP. Trickle ICE imposes no
restrictions on the way this is done (e.g., some applications may
choose not to trickle updates for server reflexive candidates and
instead rely on the discovery of peer reflexive ones).
When candidates are trickled, the signaling protocol MUST deliver
each candidate to the receiving Trickle ICE implementation not more
than once and in the same order it was conveyed. If the signaling
protocol provides any candidate retransmissions, they need to be
hidden from the ICE implementation.
Also, candidate trickling needs to be correlated to a specific ICE
session, so that if there is an ICE restart, any delayed updates for
a previous session can be recognized as such and ignored by the
receiving party. For example, applications that choose to signal
candidates via SDP may include a ufrag value in the corresponding
a=candidate line such as:
a=candidate:1 1 UDP 2130706431 2001:db8::1 5000 typ host ufrag 8hhY
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Or as another example, WebRTC implementations may include a ufrag in
the JavaScript objects that represent candidates.
Note: The signaling protocol needs to provide a mechanism for both
parties to indicate and agree on the ICE session in force (as
identified by the ufrag) so that they have a consistent view of which
candidates are to be paired. This is especially important in the
case of ICE restarts (see Section 13).
Once the candidate has been conveyed to the remote party, the agent
checks if any remote candidates are currently known for this same
stream and component. If not, the new candidate will simply be added
to the list of local candidates.
Otherwise, if the agent has already learned of one or more remote
candidates for this stream and component, it will begin pairing the
new local candidates with them and adding the pairs to the existing
check lists according to their priority.
Note: A Trickle ICE agent MUST NOT pair a local candidate until it
has been trickled to the remote agent.
8.1. Pairing Newly Learned Candidates and Updating Check Lists
Forming candidate pairs works as described in the ICE specification
[rfc5245bis]. However, actually adding the new pair to a check list
happens according to the rules described below.
If the check list where the pair is to be added already contains the
maximum number of candidate pairs (100 by default as per
[rfc5245bis]), the new pair is discarded.
If the new pair's local candidate is server reflexive, the server
reflexive candidate MUST be replaced by its base before adding the
pair to the list.
Once this is done, the agent examines the check list looking for
another pair that would be redundant with the new one. If such a
pair exists and the type of its remote candidate is not peer
reflexive, the pair with the higher priority is kept and the one with
the lower priority is discarded. If, on the other hand, the type of
the remote candidate in the pre-existing pair is peer reflexive, the
agent MUST replace it with the newly formed pair (regardless of their
respective priorities); this is done by setting the priority of the
new candidate to the priority of the pre-existing candidate and then
re-sorting the check list.
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For all other pairs, including those with a server reflexive local
candidate that were not found to be redundant, the rules specified in
the following section apply.
8.1.1. Inserting a New Pair in a Check List
Consider the following tabular representation of all check lists in
an agent (note that initially for one of the foundations, i.e., f5,
there are no candidate pairs):
+-----------------+------+------+------+------+------+
| | f1 | f2 | f3 | f4 | f5 |
+-----------------+------+------+------+------+------+
| m1 (Audio.RTP) | F | F | F | | |
+-----------------+------+------+------+------+------+
| m2 (Audio.RTCP) | F | F | F | F | |
+-----------------+------+------+------+------+------+
| m3 (Video.RTP) | F | | | | |
+-----------------+------+------+------+------+------+
| m4 (Video.RTCP) | F | | | | |
+-----------------+------+------+------+------+------+
Figure 1: Example of Check List State
Each row in the table represents a component for a given media stream
(e.g., m1 and m2 might be the RTP and RTCP components for audio).
Each column represents one foundation. Each cell represents one
candidate pair. In the foregoing table, "F" stands for "frozen"; in
the tables below, "W" stands for "waiting" and "S" stands for
"succeeded".
When an agent commences ICE processing, in accordance with
Section 5.1.2.6 of [rfc5245bis] it will unfreeze (i.e., place in the
Waiting state) the topmost candidate pair in every column (i.e., the
pair with the lowest component ID). This state is shown in the
following table, with candidate pairs in the Waiting state marked by
"W".
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+-----------------+------+------+------+------+------+
| | f1 | f2 | f3 | f4 | f5 |
+-----------------+------+------+------+------+------+
| m1 (Audio.RTP) | S | W | W | | S |
+-----------------+------+------+------+------+------+
| m2 (Audio.RTCP) | W | F | F | W | W |
+-----------------+------+------+------+------+------+
| m3 (Video.RTP) | W | | | | W |
+-----------------+------+------+------+------+------+
| m4 (Video.RTCP) | W | | | | W |
+-----------------+------+------+------+------+------+
Figure 6: Check List State with Newly Formed Pair, Case 3
Case 4: In all other cases, set the state to Frozen.
8.2. Announcing End of Candidates
Once all candidate gathering is completed or expires for an ICE
session associated with a specific media stream, the agent will
generate an "end-of-candidates" indication for that session and
convey it to the remote agent via the signaling channel. Although
the exact form of the indication depends on the application protocol,
the indication MUST specify the generation (ufrag/pwd combination) so
that an agent can correlate the end-of-candidates indication with a
particular ICE session. The indication can be conveyed in the
following ways:
o As part of an initiation request (which would typically be the
case with the initial ICE description for half trickle)
o Along with the last candidate an agent can send for a stream
o As a standalone notification (e.g., after STUN Binding requests or
TURN Allocate requests to a server time out and the agent has is
not actively gathering candidates)
Conveying an end-of-candidates indication in a timely manner is
important in order to avoid ambiguities and speed up the conclusion
of ICE processing. In particular:
o A controlled Trickle ICE agent SHOULD convey an end-of-candidates
indication after it has completed gathering for a media stream,
unless ICE processing terminates before the agent has had a chance
to complete gathering.
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o A controlling agent MAY conclude ICE processing prior to conveying
end-of-candidates indications for all streams. However, it is
RECOMMENDED for a controlling agent to convey end-of-candidates
indications whenever possible for the sake of consistency and to
keep middleboxes and controlled agents up-to-date on the state of
ICE processing.
When conveying an end-of-candidates indication during trickling
(rather than as a part of the initial ICE description or a response
thereto), it is the responsibility of the using protocol to define
methods for relating the indication to one or more specific media
streams.
Receiving an end-of-candidates indication enables an agent to update
check list states and, in case valid pairs do not exist for every
component in every media stream, determine that ICE processing has
failed. It also enables an agent to speed up the conclusion of ICE
processing when a candidate pair has been validated but it involves
the use of lower-preference transports such as TURN. In such
situations, an implementation MAY choose to wait and see if higher-
priority candidates are received; in this case the end-of-candidates
indication provides a notification that such candidates are not
forthcoming.
An agent MAY also choose to generate an end-of-candidates indication
before candidate gathering has actually completed, if the agent
determines that gathering has continued for more than an acceptable
period of time. However, an agent MUST NOT convey any more
candidates after it has conveyed an end-of-candidates indication.
When performing half trickle, an agent SHOULD convey an end-of-
candidates indication together with its initial ICE description
unless it is planning to potentially trickle additional candidates
(e.g., in case the remote party turns out to support Trickle ICE).
After an agent conveys the end-of-candidates indication, it will
update the state of the corresponding check list as explained in
Section 7.2. Past that point, an agent MUST NOT trickle any new
candidates within this ICE session. After an agent has received an
end-of-candidates indication, it MUST also ignore any newly received
candidates for that media stream or media session. Therefore, adding
new candidates to the negotiation is possible only through an ICE
restart (see Section 13).
This specification does not override regular ICE semantics for
concluding ICE processing. Therefore, even if end-of-candidates
indications are conveyed, an agent will still need to go through pair
nomination. Also, if pairs have been nominated for components and
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media streams, ICE processing MAY still conclude even if end-of-
candidates indications have not been received for all streams.
9. Receiving Additional Remote Candidates
At any time during ICE processing, a Trickle ICE agent might receive
new candidates from the remote agent. When this happens and no local
candidates are currently known for this same stream, the new remote
candidates are added to the list of remote candidates.
Otherwise, the new candidates are used for forming candidate pairs
with the pool of local candidates and they are added to the local
check lists as described in Section 8.1.
Once the remote agent has completed candidate gathering, it will
convey an end-of-candidates indication. Upon receiving such an
indication, the local agent MUST update check list states as per
Section 7.2. This might lead to some check lists being marked as
Failed.
10. Receiving an End-Of-Candidates Indication
When an agent receives an end-of-candidates indication for a specific
media stream, it will update the state of the relevant check list as
per Section 7.2. If the check list is still in the Active state
after the update, the agent will persist the fact that an end-of-
candidates indication has been received and take it into account in
future updates to the check list.
11. Trickle ICE and Peer Reflexive Candidates
Even though Trickle ICE does not explicitly modify the procedures for
handling peer-reflexive candidates, use of Trickle ICE can have an
impact on how they are processed. With Trickle ICE, it is possible
that server reflexive candidates can be discovered as peer reflexive
in cases where incoming connectivity checks are received from these
candidates before the trickle updates that carry them.
While this would certainly increase the number of cases where ICE
processing nominates and selects candidates discovered as peer-
reflexive, it does not require any change in processing.
It is also likely that some applications would prefer not to trickle
server reflexive candidates to entities that are known to be publicly
accessible and where sending a direct STUN binding request is likely
to reach the destination faster than the trickle update that travels
through the signaling path.
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This specification does not directly modify the procedures for ending
ICE processing described in Section 7 of [rfc5245bis], and Trickle
ICE implementations follow the same rules.
13. Subsequent Exchanges
Either agent MAY convey subsequent candidate information at any time
allowed by the signaling protocol in use. When this happens, agents
will use [rfc5245bis] semantics to determine whether or not the new
candidate information require an ICE restart. If an ICE restart
occurs, the agents can assume that Trickle ICE is still supported if
support was determined previously, and thus can engage in Trickle ICE
behavior as they would in an initial exchange of ICE descriptions
where support was determined through a capabilities discovery method.
14. Unilateral Use of Trickle ICE (Half Trickle)
In half trickle mode, the initiator conveys the initial ICE
description with a full generation of candidates. This ensures that
the ICE description can be processed by a regular ICE responder and
is mostly meant for use in cases where support for Trickle ICE cannot
be confirmed prior to conveying the initial ICE description. The
initial ICE description indicate support for Trickle ICE, which means
the responder can respond with something less than a full generation
of candidates and then trickle the rest. The initial ICE description
for half trickle would typically contain an end-of-candidates
indication, although this is not mandatory because if trickle support
is confirmed then the initiator can choose to trickle additional
candidates before it conveys an end-of-candidates indication.
The half trickle mechanism can be used in cases where there is no way
for an agent to verify in advance whether a remote party supports
Trickle ICE. Because the initial ICE description contain a full
generation of candidates, it can thus be handled by a regular ICE
agent, while still allowing a Trickle ICE agent to use the
optimization defined in this specification. This prevents
negotiation from failing in the former case while still giving
roughly half the Trickle ICE benefits in the latter (hence the name
of the mechanism).
Use of half trickle is only necessary during an initial exchange of
ICE descriptions. After both parties have received an ICE
description from their peer, they can each reliably determine Trickle
ICE support and use it for all subsequent exchanges.
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In some instances, using half trickle might bring more than just half
the improvement in terms of user experience. This can happen when an
agent starts gathering candidates upon user interface cues that the
user will soon be initiating an interaction, such as activity on a
keypad or the phone going off hook. This would mean that some or all
of the candidate gathering could be completed before the agent
actually needs to convey the candidate information. Because the
responder will be able to trickle candidates, both agents will be
able to start connectivity checks and complete ICE processing earlier
than with regular ICE and potentially even as early as with full
trickle.
However, such anticipation is not always possible. For example, a
multipurpose user agent or a WebRTC web page where communication is a
non-central feature (e.g., calling a support line in case of a
problem with the main features) would not necessarily have a way of
distinguishing between call intentions and other user activity. In
such cases, using full trickle is most likely to result in an ideal
user experience. Even so, using half trickle would be an improvement
over regular ICE because it would result in a better experience for
responders.
15. Requirements for Signaling Protocols
In order to fully enable the use of Trickle ICE, this specification
defines the following requirements for signaling protocols.
o A signaling protocol SHOULD provide a way for parties to advertise
and discover support for Trickle ICE before an ICE session begins
(see Section 3).
o A signaling protocol MUST provide methods for incrementally
conveying (i.e., "trickling") additional candidates after
conveying the initial ICE description (see Section 8).
o A signaling protocol MUST deliver each trickled candidate not more
than once and in the same order it was conveyed (see Section 8).
o A signaling protocol MUST provide a mechanism for both parties to
indicate and agree on the ICE session in force (see Section 8).
o A signaling protocol MUST provide a way for parties to communicate
the end-of-candidates indication, which MUST specify the
particular ICE session to which the indication applies (see
Section 8.2).
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One important aspect of regular ICE is that connectivity checks for a
specific foundation and component are attempted simultaneously by
both agents, so that any firewalls or NATs fronting the agents would
whitelist both endpoints and allow all except for the first
("suicide") packets to go through. This is also important to
unfreezing candidates at the right time. While not crucial,
preserving this behavior in Trickle ICE is likely to improve ICE
performance.
To achieve this, when trickling candidates, agents MUST respect the
order in which the components and streams appear (implicitly or
explicitly) as they have been negotiated by means of the relevant
candidate information. Therefore a candidate for a specific
component MUST NOT be conveyed prior to candidates for other
components within the same foundation. In addition, candidates MUST
be paired, following the procedures in Section 8.1.1, in the same
order they are conveyed.
For example, the following SDP description contains two components
(RTP and RTCP) and two foundations (host and server reflexive):
v=0
o=jdoe 2890844526 2890842807 IN IP6 2001:db8:a0b:12f0::1
s=
c=IN IP6 2001:db8:a0b:12f0::1
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 5000 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 2001:db8:a0b:12f0::1 5000 typ host
a=candidate:1 2 UDP 2130706431 2001:db8:a0b:12f0::1 5001 typ host
a=candidate:2 1 UDP 1694498815 2001:db8:a0b:12f0::3 5000 typ srflx
raddr 2001:db8:a0b:12f0::1 rport 8998
a=candidate:2 2 UDP 1694498815 2001:db8:a0b:12f0::3 5001 typ srflx
raddr 2001:db8:a0b:12f0::1 rport 8998
For this candidate information the RTCP host candidate MUST NOT be
conveyed prior to the RTP host candidate. Similarly the RTP server
reflexive candidate MUST be conveyed together with or prior to the
RTCP server reflexive candidate.
Similar considerations apply at the level of media streams in
addition to foundations; this is covered by the requirement to always
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[XEP-0176]
Beda, J., Ludwig, S., Saint-Andre, P., Hildebrand, J.,
Egan, S., and R. McQueen, "XEP-0176: Jingle ICE-UDP
Transport Method", XEP XEP-0176, June 2009.
Appendix A. Interaction with Regular ICE
The ICE protocol was designed to be flexible enough to work in and
adapt to as many network environments as possible. Despite that
flexibility, ICE as specified in [rfc5245bis] does not by itself
support trickle ICE. This section describes how trickling of
candidates interacts with ICE.
[rfc5245bis] describes the conditions required to update check lists
and timer states while an ICE agent is in the Running state. These
conditions are verified upon transaction completion and one of them
stipulates that:
If there is not a pair in the valid list for each component of the
media stream, the state of the check list is set to Failed.
This could be a problem and cause ICE processing to fail prematurely
in a number of scenarios. Consider the following case:
1. Alice and Bob are both located in different networks with Network
Address Translation (NAT). Alice and Bob themselves have
different address but both networks use the same private internet
block (e.g., the "20-bit block" 172.16/12 specified in
[RFC1918]).
2. Alice conveys to Bob the candidate 172.16.0.1 which also happens
to correspond to an existing host on Bob's network.
3. Bob creates a check list consisting solely of 172.16.0.1 and
starts checks.
4. These checks reach the host at 172.16.0.1 in Bob's network, which
responds with an ICMP "port unreachable" error; per [rfc5245bis]
Bob marks the transaction as Failed.
At this point the check list only contains Failed candidates and the
valid list is empty. This causes the media stream and potentially
all ICE processing to fail, even though if trickle agents could
subsequently convey candidates that would cause previously empty
check lists to become non-empty.
A similar race condition would occur if the initial ICE description
from Alice contain only candidates that can be determined as
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unreachable from any of the candidates that Bob has gathered (e.g.,
this would be the case if Bob's candidates only contain IPv4
addresses and the first candidate that he receives from Alice is an
IPv6 one).
Another potential problem could arise when a non-trickle ICE
implementation initiates an interaction with a Trickle ICE
implementation. Consider the following case:
1. Alice's client has a non-Trickle ICE implementation.
2. Bob's client has support for Trickle ICE.
3. Alice and Bob are behind NATs with address-dependent filtering
[RFC4787].
4. Bob has two STUN servers but one of them is currently
unreachable.
After Bob's agent receives Alice's initial ICE description it would
immediately start connectivity checks. It would also start gathering
candidates, which would take a long time because of the unreachable
STUN server. By the time Bob's answer is ready and conveyed to
Alice, Bob's connectivity checks may well have failed: until Alice
gets Bob's answer, she won't be able to start connectivity checks and
punch holes in her NAT. The NAT would hence be filtering Bob's
checks as originating from an unknown endpoint.
Appendix B. Interaction with ICE Lite
The behavior of ICE lite agents that are capable of Trickle ICE does
not require any particular rules other than those already defined in
this specification and [rfc5245bis]. This section is hence provided
only for informational purposes.
An ICE lite agent would generate candidate information as per
[rfc5245bis] and would indicate support for Trickle ICE. Given that
the candidate information will contain a full generation of
candidates, it would also be accompanied by an end-of-candidates
indication.
When performing full trickle, a full ICE implementation could
conveying the initial ICE description or response thereto with no
candidates. After receiving a response that identifies the remote
agent as an ICE lite implementation, the initiator can choose to not
trickle any additional candidates. The same is also true in the case
when the ICE lite agent initiates the interaction and the full ICE
agent is the responder. In these cases the connectivity checks would
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o Added section on requirements for signaling protocols.
o Clarified terminology.
o Addressed various WG feedback.
C.10. Changes from draft-ietf-ice-trickle-03
o Provided more detailed description of unfreezing behavior,
specifically how to replace pre-existing peer-reflexive candidates
with higher-priority ones received via trickling.
C.11. Changes from draft-ietf-ice-trickle-02
o Adjusted unfreezing behavior when there are disparate foundations.
C.12. Changes from draft-ietf-ice-trickle-01
o Changed examples to use IPv6.
C.13. Changes from draft-ietf-ice-trickle-00
o Removed dependency on SDP (which is to be provided in a separate
specification).
o Clarified text about the fact that a check list can be empty if no
candidates have been sent or received yet.
o Clarified wording about check list states so as not to define new
states for "Active" and "Frozen" because those states are not
defined for check lists (only for candidate pairs) in ICE core.
o Removed open issues list because it was out of date.
o Completed a thorough copy edit.
C.14. Changes from draft-mmusic-trickle-ice-02
o Addressed feedback from Rajmohan Banavi and Brandon Williams.
o Clarified text about determining support and about how to proceed
if it can be determined that the answering agent does not support
Trickle ICE.
o Clarified text about check list and timer updates.
o Clarified when it is appropriate to use half trickle or to send no
candidates in an offer or answer.
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o Updated the list of open issues.
C.15. Changes from draft-ivov-01 and draft-mmusic-00
o Added a requirement to trickle candidates by order of components
to avoid deadlocks in the unfreezing algorithm.
o Added an informative note on peer-reflexive candidates explaining
that nothing changes for them semantically but they do become a
more likely occurrence for Trickle ICE.
o Limit the number of pairs to 100 to comply with 5245.
o Added clarifications on the non-importance of how newly discovered
candidates are trickled/sent to the remote party or if this is
done at all.
o Added transport expectations for trickled candidates as per Dale
Worley's recommendation.
C.16. Changes from draft-ivov-00
o Specified that end-of-candidates is a media level attribute which
can of course appear as session level, which is equivalent to
having it appear in all m-lines. Also made end-of-candidates
optional for cases such as aggressive nomination for controlled
agents.
o Added an example for ICE lite and Trickle ICE to illustrate how,
when talking to an ICE lite agent doesn't need to send or even
discover any candidates.
o Added an example for ICE lite and Trickle ICE to illustrate how,
when talking to an ICE lite agent doesn't need to send or even
discover any candidates.
o Added wording that explicitly states ICE lite agents have to be
prepared to receive no candidates over signaling and that they
should not freak out if this happens. (Closed the corresponding
open issue).
o It is now mandatory to use MID when trickling candidates and using
m-line indexes is no longer allowed.
o Replaced use of 0.0.0.0 to IP6 :: in order to avoid potential
issues with RFC2543 SDP libraries that interpret 0.0.0.0 as an on-
hold operation. Also changed the port number here from 1 to 9
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since it already has a more appropriate meaning. (Port change
suggested by Jonathan Lennox).
o Closed the Open Issue about use about what to do with cands
received after end-of-cands. Solution: ignore, do an ICE restart
if you want to add something.
o Added more terminology, including trickling, trickled candidates,
half trickle, full trickle,
o Added a reference to the SIP usage for Trickle ICE as requested at
the Boston interim.
C.17. Changes from draft-rescorla-01
o Brought back explicit use of Offer/Answer. There are no more
attempts to try to do this in an O/A independent way. Also
removed the use of ICE Descriptions.
o Added SDP specification for trickled candidates, the trickle
option and 0.0.0.0 addresses in m-lines, and end-of-candidates.
o Support and Discovery. Changed that section to be less abstract.
As discussed in IETF85, the draft now says implementations and
usages need to either determine support in advance and directly
use trickle, or do half trickle. Removed suggestion about use of
discovery in SIP or about letting implementing protocols do what
they want.
o Defined Half Trickle. Added a section that says how it works.
Mentioned that it only needs to happen in the first o/a (not
necessary in updates), and added Jonathan's comment about how it
could, in some cases, offer more than half the improvement if you
can pre-gather part or all of your candidates before the user
actually presses the call button.
o Added a short section about subsequent offer/answer exchanges.
o Added a short section about interactions with ICE Lite
implementations.
o Added two new entries to the open issues section.
C.18. Changes from draft-rescorla-00
o Relaxed requirements about verifying support following a
discussion on MMUSIC.
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